Seems this is a common issue, unfortunately, I tried a bunch of things I found via search but no luck. I upgraded an application (including some jars) that was using javax.mail. The old version still works, but even with all the same code and parameters, the new version fails. The working version uses j2ee-1.3.1.jar, I've tried using that as well as mail.jar 1.4.2 and smtp.jar. I've dropped the default provider list in the /META-INF directory and no changes either. I've turned on debugging and here's the output of the failing verison:
I resolved the issue. I had to add both Activation.jar and Mail.jar to my classpath and remove all .class files from the javax/mail package within J2EE-1.3.1.jar that was also on the classpath. Not sure what caused the conflict.